Histidine

Abstract: We have identified a new protein fold--the alpha/beta hydrolase fold--that is common to several hydrolytic enzymes of widely differing phylogenetic origin and catalytic function. The core of each enzyme is similar: an alpha/beta sheet, not barrel, of eight beta-sheets connected by alpha-helices. These enzymes have diverged from a common ancestor so as to preserve the arrangement of the catalytic residues, not the binding site. They all have a catalytic triad, the elements of which are borne on loops which are the best-conserved structural features in the fold. Only the histidine in the nucleophile-histidine-acid catalytic triad is completely conserved, with the nucleophile and acid loops accommodating more than one type of amino acid. The unique topological and sequence arrangement of the triad residues produces a catalytic triad which is, in a sense, a mirror-image of the serine protease catalytic triad. There are now four groups of enzymes which contain catalytic triads and which are related by convergent evolution towards a stable, useful active site: the eukaryotic serine proteases, the cysteine proteases, subtilisins and the alpha/beta hydrolase fold enzymes.

Abstract: Zinc is required for the activity of > 300 enzymes, covering all six classes of enzymes. Zinc binding sites in proteins are often distorted tetrahedral or trigonal bipyramidal geometry, made up of the sulfur of cysteine, the nitrogen of histidine or the oxygen of aspartate and glutamate, or a combination. Zinc in proteins can either participate directly in chemical catalysis or be important for maintaining protein structure and stability. In all catalytic sites, the zinc ion functions as a Lewis acid. Researchers in our laboratory are dissecting the determinants of molecular recognition and catalysis in the zinc-binding site of carbonic anhydrase. These studies demonstrate that the chemical nature of the direct ligands and the structure of the surrounding hydrogen bond network are crucial for both the activity of carbonic anhydrase and the metal ion affinity of the zinc-binding site. An understanding of naturally occurring zinc-binding sites will aid in creating de novo zinc-binding proteins and in designing new metal sites in existing proteins for novel purposes such as to serve as metal ion biosensors.